As a valve system for a multi-cylinder engine, there is known a valve system that includes, for one valve of each cylinder, a plurality of cams having different shapes of nose sections and selects a cam for opening and closing the valve out of the cams to make it possible to switch valve opening amounts, valve opening and closing periods, and the like of intake and exhaust valves according to an operation state of the engine.
For example, Patent Literature 1 discloses a valve system including a cam shaft including a shaft section and a cylindrical cam element section movably spline-fitted on the shaft section in an axial direction. In the valve system, on the outer circumference of the cam element section, a plurality of adjacent cams having different shapes of nose sections are provided for one valve. The valve system moves the cam element section in the axial direction to thereby switch a cam that opens and closes the valve.
Specifically, in the valve system disclosed in Patent Literature 1, a plurality of cam element sections are provided to correspond to cylinders of a multi-cylinder engine. End face cams are formed on both end faces of the cam element section. The valve system disclosed in the literature includes operation members retractably provided with respect to opposing positions of the end face cams. The operation members are driven by an actuator to project and engage with the end face cams when projecting. Consequently, the cam element section is moved in the axial direction and the cam is switched.
Incidentally, in the valve system disclosed in Patent Literature 1, the end face cams are provided on both the end faces of the cam element section in order to move the cam element section to both sides in the axial direction of the shaft section. The valve system includes the operation member for each of the end face cams on both the sides. Therefore, two operation members are necessary for one cam element section. The number of components increases.
As measures against this problem, in order to reduce the number of components of the operation members, it is conceivable to adopt a method of disposing a single operation member between cylinders adjacent to each other, projecting the operation member to between opposed end face cams of two cam element sections disposed closer to each other, and engaging the operation member with the respective end face cams to thereby separate both the cam element sections in the axial direction and switch a cam.
However, with this method, there is a problem in that, between two cylinders disposed adjacent to each other and discontinuous in ignition order, a period in which the operation member can be projected to between the opposed end face cams decreases and, in particular, during high-speed rotation of an engine, it is difficult to switch the cam. A reason why the period in which the operation member can be projected decreases is explained below using a four-cylinder engine as an example.
For example, in a four-cylinder engine in which first, second, third, and fourth cylinders are disposed in this order, when ignition order is the order of the third cylinder, the fourth cylinder, the second cylinder, and the first cylinder, the second and third cylinders are adjacent to each other but are discontinuous in the ignition order. In order to move a cam element section while a valve is closed, lift sections of end face cams are disposed to overlap a projecting position of an operation member (overlap the operation member after projection in axial direction view) when nose sections do not open and close the valve. That is, both of the lift sections and reference planes (parts that are not the lift sections) of the end face cams are set on the basis of phases of the nose sections.
Since the second and third cylinders are discontinuous in the ignition order, the phases of the nose sections of two end face cams opposed between these cylinders are not in a continuous relation. As a result, an angle range in which two reference planes of the two end face cams overlap is divided into two ranges, which are respectively narrow angle ranges.
Incidentally, between the cylinders adjacent to each other, timing when the operation members can be projected is limited to time when each of the reference planes of the opposed two end face cams overlaps the projecting position of the operation member. On the other hand, as explained above, when the adjacent cylinders are discontinuous in the ignition order, compared with when the cylinders are continuous in the ignition order, an angle range in which the two reference planes overlap is narrow. Therefore, between the cylinders discontinuous in the ignition order, a period in which the operation member can be projected is short.
Moreover, during high-speed rotation of the engine, since rotating speed of a cam shaft is high, the period in which the operation member can be projected is shorter. As a result, depending on projection speed of the operation member, even if it is attempted to project the operation member to between the two reference planes, the projection is late. It is difficult to switch a cam section.